The present invention relates to a tuneable filter, a signal receiving or transceiving device comprising a tuneable filter, and to a method of calibrating a tuneable filter. The tuneable filter may be, in particular but not exclusively, an antenna filter.
Currently produced transceivers used in cordless and cellular telephones typically comprise two principal integrated circuits (ICs) one of which contains the radio front end and the other of which contains the digital processing and is sometimes referred to as the xe2x80x9cback-endxe2x80x9d. The front-end radio IC must amplify weak, radio frequency (RF) signals, hence the transistors are operating in their linear region and amplify with minimum distortion. Normally the front-end radio IC is implemented in a fabrication process with bipolar devices as these provide superior performance when operating at RF compared to CMOS devices (in terms of amplification, distortion, linearity and noise). For the back-end IC, a CMOS process is generally used because these devices provide a very low power consumption and a high packing density.
In choosing to fabricate a single chip radio the architecture may require RF components to be integrated on a digital CMOS IC. This means that CMOS transistors are used instead of bipolar transistors for the RF front-end circuits. Processes such as the Philips CMOS 18 (or C18), where 18 refers to the minimum length of a device viz. 0.18 xcexcm, have been characterised for RF.
A disadvantage of having to use a CMOS process is that analogue passive components have larger tolerances than either discrete devices or those fabricated using bipolar transistor processes. The tolerances are frequently batch dependent so that the values of the passive components may vary from a true value in the same direction. Such passive components are used in circuits such as oscillators and filters to provide frequency selectivity but the relatively large tolerances will lead to a large range of values being possible risking the frequency selective circuits being tuned too far from the correct value. This may cause the wanted RF signals to be heavily attenuated or distorted, so rendering the entire receiver chip unusable, resulting in a poor yield and a consequent unacceptably high cost of production.
U.S. Pat. specification No. 6,081,700 discloses a radio having a self-tuning antenna and a method of tuning the antenna by way of applying a control voltage to a varactor diode so that the antenna can achieve a peak or predetermined quality level. A controller, which may be a DSP (digital signal processor), generates a calibration signal which is radiated within the radio. The signal is received at the antenna and is processed by receiver circuitry. The controller measures the quality of received signal and the result of the measurement is used to control the tuning of the antenna. The quality metric which may be measured can be either the received signal strength indicator (RSSI), bit error rate (BER) or signal to noise ratio.
For a single channel radio receiver the value of the control voltage applied to the varactor may be stored and subsequently updated to re-peak or retune the receiver. Values of control voltages for tuning the antenna for a plurality of radio channels may also stored for subsequent use. Also in order to enable the antenna to operate over a band of channels, the band can be characterised by control voltages at various points in the operating band and that information can be stored and either utilised for tuning the antenna to cover sub-band segments or to generate tuning voltages interpolated from the stored information. The measurement of tuning voltages for a plurality of radio channels requires the generation of a calibration signal for each of the radio channels and a period of time to evaluate the optimum tuning voltage.
An object of the present invention is to provide an improved tuneable filter, an improved device comprising a tuneable filter and an improved method of calibrating a tuneable filter.
According to a first aspect of the present invention there is provided a signal receiving device comprising a tuneable filter and a control means, wherein the tuneable filter comprises an input for applying a signal at a first frequency, an output for delivering the filtered signal and a variable reactance means, and wherein the control means comprises means for generating a control signal for varying the reactance of the variable reactance means, measurement means for measuring a post-filter signal quality, first storage means comprising data relating the degree of off-tune of the filter to a consequent signal quality, and second storage means for storing an indication of a control signal value and an indication of a frequency, wherein the control means is operable to tune the filter to the input signal at the first frequency by determining a first value of the control signal which produces a peak signal quality, to off-tune the filter from the first frequency by selecting a second value of the control signal, to measure the off-tune signal quality, to determine a peak frequency of the off-tune filter by using the peak signal quality, the measured off-tune signal quality and the data relating the degree of off-tune to the consequent signal quality, and to store an indication of the second value of the control signal and an indication of the peak frequency of the off-tune filter.
According to a second aspect of the present invention there is provided a transceiver device comprising a receiver part and a transmitter part, wherein the receiver part comprises a signal receiving device in accordance with the first aspect of the invention.
According to a third aspect of the present invention there is provided a tuneable filter comprising a variable reactance means, wherein the variable reactance means comprises a bank of selectable capacitors.
According to a fourth aspect of the present invention there is provided a method of calibrating a tuneable filter having a variable reactance means, the reactance of the variable reactance means being controllable by a control signal, the method comprising supplying the tuneable filter with a signal having a first frequency, tuning the filter by determining a first value of the control signal which produces a peak post-filter signal quality, off-tuning the filter by selecting a second value of the control signal and measuring the resulting off-tune signal quality, determining a peak frequency of the off-tune filter by using the peak signal quality, the off-tune signal quality and data relating the degree of off-tune to the consequent signal quality, and storing an indication of the second value of the control signal and an indication of the peak frequency of the off-tune filter.
By requiring a signal at only a single frequency for calibration of the tuneable filter, the generation of calibration signals at a plurality of frequencies is avoided. By employing off-tune measurements of signal quality and using data relating the degree of off-tune to the consequent signal quality, the time required for determining the optimum tuning is reduced.
Optionally the filter may be calibrated for a plurality of frequencies by off-tuning the filter by various amounts using various values of the control signal, measuring, for each value of the control signal, the resulting off-tune signal quality and determining the corresponding peak frequency of the off-tune filter, and storing an indication of each value of the control signal and a corresponding indication of the peak frequency of the off-tune filter.
The stored indications may be used to determine further values of the control signal and corresponding peak frequencies by interpolation or extrapolation, and optionally the further values may be stored.
In operation, after calibration, the stored indications may be used to select an appropriate value of the control signal for operating on a particular frequency. In particular, for operation in a frequency hopped system, the control signal may be selected for optimum signal quality on each frequency.
Optionally the variable reactance means may comprise a bank of switchable inductors or a bank of switchable capacitors.
Optionally the signal quality may be one of signal strength, signal to noise ratio, bit error rate, timing jitter, and data eye opening.